1. Field of the Invention
The present invention relates to an optical communications, and more particularly, to a low cost bidirectional optical transceiver having minimal component alignment criteria.
2. Description of the Related Art
Generally, in optical communication utilizing optical fibers a bidirectional optical transceiver transmits a plurality of channels simultaneously using different wavelengths of light. In the case of a single-mode optical fiber, the communications typically occur at two optical wavelengths (e.g., 1.3 nm and 1.55 nm), because these wavelengths exhibit less attenuation. An FTTH (fiber-to-the-home) system is a WDM (wavelength division multiplex) system that makes it possible to integrate a communication channel and a CATV channel onto a line of optical fiber. Thus, rather than using separate lines of optical fiber for transmission and reception, a single line of optical fiber is used to perform transmission and reception at the same time. This saves the cost for installing optical fibers, decreases the number of optical components, and implements a more economical optical communication system.
Conventional bidirectional optical transceivers are classified into those using a PLC (planar lightwave circuit) substrate and those using a TO-Can substrate. FIG. 1 shows a cross-sectional view of a conventional bidirectional optical transceiver having two TO-Cans. The bidirectional optical transceiver 1 is composed of a wavelength distributor 6 adapted to distribute optical signals inputted from external sources, as well as optical signals inputted/outputted to/from a laser diode (LD) 3 and a photodetector (PD) 4 and a transceiver module made of a group of optical fibers 5, which are light-receiving devices adapted to identify respective wavelengths distributed from the wavelength distributor 6.
The wavelength distributor 6 is composed of a wavelength distributor 6 adapted to distribute the optical signals provided by LD 3, as well as those from external sources, and a Y-distributor (not shown) acting as a transmission/reception line for the optical signals. The wavelength distributor 6 is interposed in between the transceiver module and the Y-distributor. The wavelength distributor 6, may include a wavelength division multiplex filter, that performs multiple division of inputted optical signals according to their wavelength bands. In this case, a multiplexer filter, or a Bragg diffraction grating may be used.
The Y-distributor is an optical waveguide having a common waveguide acting as a line both for optical signals inputted from external sources and for those provided by LD 3. The transceiver module is adapted to modulate inputted signals into optical signals and an optical detector (not shown) that is adapted to detect optical signals inputted via an optical fiber, which monitors the optical intensity of the LD 3.
The bidirectional optical transceiver 1 has a light-transmitting portion and a light-receiving portion, each of which is made of a TO-Can 2, that are actively aligned, and coupled to each other. Since the TO-Can 2 itself is fabricated using common technology, the bidirectional optical transceiver has little manufacturing cost and, thus, a high yield.
However, conventional bidirectional optical transceivers become bulky. For example, in the case of a triplexer, three TO-Cans are used and coupled together. Compared with a diplexer, which uses two TO-Cans, the triplexer needs additional time and effort to actively align the additional TO-Can. Further as there are an increased number of components, the chance of a defect occurring increases and, hence, the manufacturing yield is decreased.
FIG. 2 shows a conventional bidirectional optical transceiver 10 using a PLC substrate 20. The bidirectional optical transceiver 10 comprises a connector 30 formed on a PLC substrate 20, a PD 40, an LD 50, and an optical signal monitor (not shown) adapted to monitor the intensity of light outputted from the LD 50. A waveguide 60 is formed on the PLC substrate 20 in a Y-branch structure that is bifurcated into the PD 40 and the LD 50, respectively. The PD 40 detects optical signals inputted via the waveguide 60. The LD 50 generates a predetermined wavelength of light, which is outputted external to the device through the waveguide 60. Each of the bifurcated parts of the waveguide 60, which has a Y-branch structure, is located opposite to each of the PD 40 and the LD 50, respectively.
The bidirectional optical transceiver 10 has an L-shaped housing 70, in which the PLC substrate 20 is seated. An optical fiber 80 is positioned on a side of the housing 70 and a connector 30 is positioned in a location opposite to the optical fiber 80. The bidirectional optical transceiver 10 may be configured as a bidirectional optical transceiver module, wherein the optical fiber 80 is mounted on the PLC substrate 20 and is manually aligned with an end of the waveguide 60.
Such a bidirectional optical transceiver, which has manual alignment structure, is difficult to fabricate, because the photodetector, the laser diode and the optical signal monitor must be precisely aligned, typically within an error range of 1-2 μm, and fastened on the single PLC substrate. In addition to such a high precision requirement, the optical devices should be fabricated to fulfill specific requirements. For example, the laser diode must be fabricated so that the light exits with a very small exit angle; otherwise, high insertion loss occurs. Further, when the laser diode is a Distributed Feedback Laser (DFB), there is no place left to include an optical isolator. Furthermore, the photodetector needs to be tailored to the optical waveguides. In summary, the cost of the conventional process to fabricate the optical transceiver is further increased due to the requirements for special laser diode and photodetector, and the yield is poor.
Hence, there is a need in the industry for a bidirectional optical transceiver that is compact in size and has a simple manufacturing process.